Ultrathin fluid films confined to a chemically heterogeneous slit-shaped nanopore

Abstract

The properties of a molecularly thin film of Lennard-Jones (LJ) (12,6) fluid confined to a chemically heterogeneous slit-shaped pore were investigated by the grand canonical ensemble Monte Carlo (GCEMC) method. The slit-shaped pore comprises two identical plane-parallel solid substrates, each of which consists of alternating strips of LJ(12,6) solid of two types: strongly adsorbing and weakly adsorbing. With substrates aligned so that strips of the same type oppose each other, GCEMC was used to compute equilibrium properties of the film as functions of the distance ${s}_{z}$ between the substrates. Results are compared for two well depths ${\ensuremath{\epsilon}}_{\mathrm{fs}}$ of the LJ(12,6) potential between molecules in the film and those in the strongly adsorbing strip. Variations in tensions, mean film density, and isothermal compressibility as functions of ${s}_{z}$ are correlated with structural changes in the film and with its phase behavior. In both cases, when the substrates are sufficiently close together, liquid bridges exist between the opposing strong strips, surrounded by dilute gas over the weak ones. The stronger substrate (i.e., the one with the greater value of ${\ensuremath{\epsilon}}_{\mathrm{fs}}$) is capable of stabilizing a liquidlike phase that fills the whole pore over a certain range of ${s}_{z},$ then abruptly evaporates beyond a critical value of ${s}_{z},$ to leave nanodroplets clinging to the strong strips. For the weaker substrate, however, the liquid bridges collapse to form nanodroplets directly, without the intermediate appearance of a liquid pore-filling phase.